Do I need to change my CAD model to allow for painting?
Generally it is not necessary to modify the CAD data specifically to allow for painting. Critical areas can be masked if necessary but most model makers will dress between coats and keep paint build up under control. (It is also worth noting that the paint thickness is small compared with the accuracy of most RP produced parts)
Where multiple colours are required on the same part (or textures) it is worth discussing the strategy with the model maker. Splitting the part to allow each area to be painted without masking and then assembled can save time.
Where there are small diameter holes these should be discussed in advance, as there can be a fine line between getting paint into the hole and blocking it entirely!
Generally it is better to paint both surfaces of a model, even if it is only the A surface that is polished as this will reduce the risk of distortion.